The present invention relates to a high pressure discharge lamp ballast for a high-intensity, high-pressure discharge lamp such as a high-pressure mercury lamp or a metal halide lamp, and an illumination system including an illumination fixture which makes use of the high pressure discharge lamp ballast.
FIG. 10 shows a conventional example of an electronic high pressure discharge lamp ballast. A lighting circuit 1 may be defined as including a full-wave rectifying circuit DB, a step-up chopper circuit 11 (also referred to herein as a power factor correction circuit 11, or “PFC” 11) and a polarity inverting step-down chopper circuit 12 (also referred to herein as a resonant inverter circuit 12 or merely “inverter” 12). The inverter 12 is configured by connecting an inductor L2 in series with a load and a capacitor C3 in parallel with the load to outputs of switching elements Q3 to Q6 arranged in a full bridge configuration. The switching elements Q3 to Q6 are controlled by a switching control circuit 4 (also referred to herein more generally as a “controller” 4) and operate so as to produce a high-frequency output at startup/ignition and a low-frequency rectangular output during steady-state operation. The ballast further includes a starting circuit 2 formed of a resonance boost circuit inserted between an output of the inverter 12 and a high-pressure discharge lamp DL.
FIG. 11 schematically shows an example of an operational waveform in association with the circuit of FIG. 10. In the figure, Vla refers to a lamp voltage applied across the high-pressure discharge lamp DL and Ila refers to a lamp current flowing to the high-pressure discharge lamp DL. In an A1 phase defining a starting period and further associated with a first mode of operation for the ballast, a high-frequency high voltage is applied across the high-pressure discharge lamp DL by a resonance boost effect of the starting circuit 2. When a dielectric breakdown occurs between lamp electrodes in the A1 phase, the lamp current Ila starts to flow. At this time, the flowing lamp current I1a has relatively small amplitude. This current maintains glow discharge and thereby functions to heat the electrodes. After a predetermined period of time associated with the A1 phase, operation shifts to an A3 phase further defining a steady-state lighting period, and a low-frequency rectangular wave voltage is applied to the high-pressure discharge lamp DL.
FIG. 12 also shows an example of the operational waveform in association with the circuit of FIG. 10 in greater detail. First, in the A1 phase during startup, since a pair of the switching elements Q3, Q6 and a pair of the switching elements Q4, Q5 in the inverter 12 are alternately turned on/off with a high frequency of a resonance frequency (or an integral sub-multiple thereof), the starting circuit 2 formed of the resonance boost circuitry generates a high-frequency voltage of high amplitude, thereby causing dielectric breakdown between the electrodes of the high-pressure discharge lamp DL. When dielectric breakdown occurs between the electrodes in the A1 phase the lamp current Ila starts to flow, but an operational frequency fa1 remains the same as before the dielectric breakdown, and the amplitude of the lamp current Ila is relatively small.
When the control circuit after the predetermined period of time shifts from the A1 phase to the A3 phase for steady-state lighting operation, the switching elements Q3, Q4 are alternately turned on/off with a low frequency. Then, by turning on/off the switching element Q6 with a high frequency while the switching element Q3 is turned on and turning on/off the switching element Q5 with a high frequency while the switching element Q4 is turned on, a low-frequency rectangular wave AC voltage is supplied to the high-pressure discharge lamp DL. In the A3 phase a lamp output detection circuit 3 detects the lamp voltage Vla, and in response to a detection signal provided by the detection circuit 3, the switching control circuit 4 controls an ON-time for the switching elements Q5, Q6 so as to produce an appropriate lamp current Ila. Thus, a DC power source Vdc is converted into a rectangular wave AC voltage which is necessary for stable lighting of the high-pressure discharge lamp DL, and the AC voltage is applied to the high-pressure discharge lamp La.
Therefore, in a manner previously known in the art a high voltage is generated from startup to steady-state operation of the high-pressure discharge lamp DL, thereby switching between the A1 phase as an ignition phase for generating dielectric breakdown between the electrodes and the A3 phase as a steady-state phase for maintaining arc discharge.
FIG. 13 shows transition of the lamp voltage V1a and an operating frequency f after powering on in another control example as previously known in the art. In the figure, 0 to t2 refers to the A1 phase, t2 to t3 refers to the A2 phase and t3 and thereafter refers to the A3 phase. In the control example shown, when the operating frequency is gradually lowered after power-on and reaches a frequency which is one third of the resonance frequency of a resonance circuit (fo/3) at the time t1, the frequency is fixed and a high-frequency generating operation using a resonance effect is maintained up to the time t2. After that, in periods of t2 to t2′ and t2′ to t3, the operating frequency is lowered in a stepped manner. Thereby, as shown in FIG. 14, the lamp current Ila can be increased as the operating frequency f decreases, and thus the electrodes of the high-pressure discharge lamp can be sufficiently heated. Although the same operation is performed as is shown for example in FIG. 12 from the time t3 and thereafter, since the electrodes are sufficiently heated the lamp is less likely in this case to be undesirably extinguished.
The example as shown and previously known in the art has the following problems. As shown in FIG. 11 and FIG. 12, it is desired that when the high-pressure discharge lamp is ignited in the A1 phase, the high-pressure discharge lamp shifts from glow discharge to arc discharge in the remaining A1 phase. However, since the current amplitude is small, the A1 phase shifts to the A3 phase before the electrodes of the high-pressure discharge lamp are sufficiently heated. As a result the discharge lamp may be easily extinguished and remain unlit. Furthermore, since the timing of dielectric breakdown of the high-pressure discharge lamp varies depending on the state of the high-pressure discharge lamp (i.e., a characteristic of the lamp output), a remaining electrode heating time in the A1 phase after dielectric breakdown also becomes irregular, and the high-pressure discharge lamp may easily and disadvantageously be extinguished during a time when the polarity of the high-pressure discharge lamp is inverted in the A3 phase.
In another example as previously known in the art, referring to FIG. 15, an additional operating mode (i.e., an A2 phase) for lowering the operating frequency in a stepped manner is inserted between the A1 phase and the A3 phase to overcome insufficient heating of the electrodes of the high-pressure discharge lamp by increasing the lamp current Ila in the A2 phase. It is possible in such a manner to sufficiently heat the electrodes of the high-pressure discharge lamp and shift to the A3 phase in a stable arc discharge state. However, as shown in FIG. 16, since a time required to heat the electrodes of the high-pressure discharge lamp (for example, one second or more) is previously set as the A2 phase, when the high-pressure discharge lamp does not ignite in the A1 phase the A2 phase is unnecessary, and in fact undesirable, as a starting time of the high-pressure discharge lamp becomes longer. A high voltage, though lower than the voltage in the A1 phase, is further generated in the A2 phase while the discharge lamp remains unlit, and therefore stresses are undesirably exerted on various circuit components.